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The phosphatidylinositol 3kinase (PI3K)/protein kinase B (Akt)/mammalian target of rapamycin (mTOR) signaling pathway is a major regulator of tumor cell growth, proliferation and survival. Hotspot mutations of PIK3CA are common in all subtypes of breast cancer with a prevalence of approximately 30%. GDC-0077 is a potent ATP-competitive inhibitor of PI3K alpha (IC50 = 0.038 ± 0.003 nM), thereby inhibiting the phosphorylation of membranebound PIP2 to PIP3. Biochemically, GDC-0077 is more than 300fold selective over the other Class I PI3K isoforms, beta, delta, and gamma, and more than 2000-fold more selective over PI3K class II and III family members. Importantly, GDC-0077 is more selective for mutant versus wild-type PI3K alpha in cell-based assays. Mechanism of action studies indicate that GDC-0077 treatment leads to degradation of the mutant PI3K alpha protein in a proteasome dependent fashion, without changing WT PI3K alpha levels. Drug treatment results in sustained reduction of PI3K pathway biomarkers pAkt and pPRAS40, inhibition of cell proliferation, and increased apoptosis in human PIK3CA mutant breast cancer cell lines, to a greater extent than other non-degrading inhibitors such as alpelisib. Combination studies of GDC-0077 with the CDK4/6 inhibitor palbociclib demonstrate a stronger effect in estrogen-depleted HR+ PIK3CA mutant breast cell lines, compared to cells grown in the presence of estrogen. Daily dosing of GDC-0077 in PIK3CA mutant breast cancer patient derived xenograft (PDX) models (n=4) resulted in tumor regressions, induction of apoptosis and a reduction of pAkt, pPRAS40, and pS6RP in a dose-dependent fashion. In vivo combination efficacy studies of GDC-0077 with fulvestrant and palbociclib also indicate the triple combination results in the greatest efficacy. Collectively, our preclinical data support evaluation of GDC-0077 in a triple combination with endocrine therapy and CDK4/6 inhibition in the clinic, in patients with locally advanced or metastatic hormone receptor positive breast cancer whose tumors harbor mutant PIK3CA. Citation Format: Kyle Edgar, Rebecca Hong, Kyung Song, Steven Schmidt, Marc Hafner, Alfonso Arrazate, Erin Williams, Cecile De La Cruz, Jason Oeh, Deepak Sampath, Steve Staben, Lori Friedman. GDC-0077 is a selective PI3K alpha inhibitor with robust efficacy in PIK3CA mutant hormone-positive breast cancer models [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-11-23.

PIK3CA is one of the most frequently mutated oncogenes; the p110a protein it encodes plays a central role in tumor cell proliferation. Small-molecule inhibitors targeting the PI3K p110a catalytic subunit have entered clinical trials, with early-phase GDC-0077 studies showing antitumor activity and a manageable safety profile in patients with PIK3CA-mutant breast cancer. However, preclinical studies have shown that PI3K pathway inhibition releases negative feedback and activates receptor tyrosine kinase signaling, reengaging the pathway and attenuating drug activity. Here we discover that GDC-0077 and taselisib more potently inhibit mutant PI3K pathway signaling and cell viability through unique HER2-dependent mutant p110a degradation. Both are more effective than other PI3K inhibitors at maintaining prolonged pathway suppression. This study establishes a new strategy for identifying inhibitors that specifically target mutant tumors by selective degradation of the mutant oncoprotein and provide a strong rationale for pursuing PI3Kα degraders in patients with HER2-positive breast cancer. Significance: The PI3K inhibitors GDC-0077 and taselisib have a unique mechanism of action; both inhibitors lead to degradation of mutant p110a protein. The inhibitors that have the ability to trigger specific degradation of mutant p110a without significant change in wild-type p110a protein may result in improved therapeutic index in PIK3CA-mutant tumors. See related commentary by Vanhaesebroeck et al., p. 20. This article is highlighted in the In This Issue feature, p. 1

The phosphatidylinositol 3 kinase (PI3K)/AKT/ mechanistic target of rapamycin (mTOR) signaling pathway is a major regulator of tumor cell growth, proliferation and survival. Dysregulation of the PI3K/AKT/mTOR signaling pathway through multiple different mechanisms has been described in solid tumor malignancies, including activating and transforming mutations and amplification of PIK3CA, that encodes the p110alpha subunit of PI3K. Indeed, PIK3CA hotspot mutations are highly prevalent in breast cancer, occurring in approximately 40% of HR+ tumors. The clinical candidate GDC-0077 is a potent inhibitor of PI3Kalpha (IC50 = 0.038 nM) and exerts its activity by binding to the ATP binding site of PI3K, thereby inhibiting the phosphorylation of PIP2 to PIP3. Biochemically, GDC-0077 is >300-fold more selective for PI3Kalpha over the other class I PI3K isoforms (beta, delta, and gamma) and >2000-fold more selective over PIK family members. Furthermore, GDC-0077 is more selective for mutant versus wild-type PI3Kalpha in cell based assays. The improved biochemical selectivity of GDC-0077 relative to PI3Kdelta translated in human CD69+ B-cells, which are primarily dependent on PI3Kdelta for proliferation and survival, and were more sensitive (based on reduction of cell number) to the PI3Kalpha/delta selective inhibitor taselisib (GDC-0032) than to GDC-0077. Mechanism of action (MOA) studies indicate that GDC-0077 selectively degrades mutant PI3Kalpha in a proteasome-dependent fashion resulting in reduction of PI3K pathway activity biomarkers such as pAKT and pPRAS40, inhibition of cell proliferation, and increased apoptosis in human PIK3CA-mutant breast cancer cell lines to a greater extent when compared to PIK3CA wild-type cells. In vivo, oral daily treatment of patient-derived PIK3CA-mutant breast cancer xenograft models with GDC-0077 resulted in tumor regressions, induction of apoptosis, and a reduction of pAKT, pPRAS40, and pS6RP in a dose-dependent fashion. In vivo efficacy in a PIK3CA-mutant human breast cancer xenograft model was also improved when GDC-0077 was combined with therapies for hormone-receptor positive (HR+) breast cancer such as anti-estrogens (fulvestrant) or a CDK4/6 inhibitor (palbociclib). Collectively, preclinical studies provide rationale for evaluating GDC-0077, a PI3Kalpha selective inhibitor that degrades mutant p110alpha protein, as a single agent and in combination with endocrine and targeted therapies that may provide additional benefit to patients with locally advanced or metastatic hormone receptor+ breast cancers that harbor PIK3CA mutations. Citation Format: Hong R, Edgar K, Song K, Steven S, Young A, Hamilton P, Arrazate A, De La Cruz C, Chan C, Pang J, Salphati L, Belvin M, Nannini M, Staben S, Friedman L, Sampath D. GDC-0077 is a selective PI3Kalpha inhibitor that demonstrates robust efficacy in PIK3CA mutant breast cancer models as a single agent and in combination with standard of care therapies [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr PD4-14.

Summary: We examined hematopoietic protein kinase 1 (HPK1), whose reliance on scaffold versus kinase functions for negative immune cell regulation is poorly understood and critical to its assessment as a viable drug target. We identify kinase-dependent roles for HPK1 in CD8 T cells that restrict their anti-viral and anti-tumor responses by using HPK1 kinase-dead (HPK1.kd) knockin mice. Loss of HPK1 kinase function enhanced T cell receptor signaling and cytokine secretion in a T-cell-intrinsic manner. In response to chronic lymphocytic choriomeningitis virus (LCMV) infection or tumor challenge, viral clearance and tumor growth inhibition were enhanced in HPK1.kd mice, accompanied by an increase in effector CD8 T cell function. Co-blockade of PD-L1 further enhanced T effector cell function, resulting in superior anti-viral and anti-tumor immunity over single target blockade. These results identify the importance of HPK1 kinase activity in the negative regulation of CD8 effector functions, implicating its potential as a cancer immunotherapy target. : HPK1 is implicated in several important steps that limit T cell responsiveness, but the mechanism is poorly characterized. Hernandez et al. demonstrate that HPK1’s kinase activity is essential for attenuating T cell function. Loss of HPK1 kinase activity enhanced antitumoral responses, proving to be an attractive target for cancer immunotherapy. Keywords: HPK1, hematopoietic progenitor kinase 1, cancer immunotherapy

The phosphoinositide 3-kinases (PI3Ks) are lipid kinases that activate the PI3K signaling pathway and play an essential role in regulating breast tumor cell growth, migration, and survival. Activating and transforming mutations in the PIK3CA gene (PI3K alpha) are commonly found in HER2+ breast cancer and ER+ breast cancer. GDC-0032 is a selective, orally bioavailable inhibitor with a Ki of 0.29 nM for PI3K alpha with 30 fold less inhibition of PI3K beta. In addition, GDC-0032 has increased single agent activity against PI3K alpha mutant and HER2 amplified cancer models in vitro and in vivo. The aim of our study was to evaluate the efficacy of GDC-0032 in breast cancer models in combination with standard of care therapeutics including taxanes, endocrine therapies, and HER2 targeted therapies. We evaluated cell viability for a range of dose concentrations of GDC-0032, as single agent and in combinations, in eleven breast cancer cell lines that harbor mutations in PI3K alpha (K111N, E545K, H1047R) and/or over-expressed HER2. GDC-0032 was combined with taxanes (paclitaxel and docetaxel), endocrine therapies (fulvestrant and tamoxifen) or anti-HER2 agents (trastuzumab, pertuzumab or ado-trastuzumab-emtansine) and synergy quantified using the Chou and Talalay method of Combination Index (C.I.). The combination of GDC-0032 with taxanes, endocrine therapies or anti-HER2 therapies were synergistic in the breast cancer cell lines tested based on C.I. values of less than 1.0. The combination of GDC-0032 with docetaxel in vitro resulted in decreased viability as a result of increased cell death. The in vitro results translated in vivo as GDC-0032 enhanced the anti-tumor activity of docetaxel and paclitaxel in MCF7 (E545K) xenografts that resulted in increased tumor regressions when GDC-0032 was dosed daily or intermittently on the same schedule as the taxane. In addition, the combination of GDC-0032 and fulvestrant or tamoxifen resulted in greater efficacy in the MCF7 xenograft model when compared to either agent alone. Enhanced efficacy was also observed when GDC-0032 was combined with trastuzumab or ado-trastuzumab-emtansine in the HER2+ BT474M1 (K111N) xenograft model. Moreover, the triple combinations of GDC-0032/trastuzumab/pertuzumab or GDC-0032/trastuzumab/docetaxel resulted in durable tumor regressions that were sustained in the HER2+ KPL-4 (H1047R) and BT474M1 xenograft models, respectively. All drug combinations with GDC-0032 were well tolerated in vivo based on minimal changes in body weight. Collectively, our preclinical data demonstrate that GDC-0032 enhances the efficacy of standard of care therapeutics in PI3K alpha mutant breast cancer models and provides a strong rationale for further evaluation in patients. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P4-15-02.

Purpose: We describe the preclinical pharmacology and antitumor activity of GDC-0068, a novel highly selective ATP-competitive pan-Akt inhibitor currently in clinical trials for the treatment of human cancers. Experimental Design: The effect of GDC-0068 on Akt signaling was characterized using specific biomarkers of the Akt pathway, and response to GDC-0068 was evaluated in human cancer cell lines and xenograft models with various genetic backgrounds, either as a single agent or in combination with chemotherapeutic agents. Results: GDC-0068 blocked Akt signaling both in cultured human cancer cell lines and in tumor xenograft models as evidenced by dose-dependent decrease in phosphorylation of downstream targets. Inhibition of Akt activity by GDC-0068 resulted in blockade of cell-cycle progression and reduced viability of cancer cell lines. Markers of Akt activation, including high-basal phospho-Akt levels, PTEN loss, and PIK3CA kinase domain mutations, correlate with sensitivity to GDC-0068. Isogenic PTEN knockout also sensitized MCF10A cells to GDC-0068. In multiple tumor xenograft models, oral administration of GDC-0068 resulted in antitumor activity ranging from tumor growth delay to regression. Consistent with the role of Akt in a survival pathway, GDC-0068 also enhanced antitumor activity of classic chemotherapeutic agents. Conclusions: GDC-0068 is a highly selective, orally bioavailable Akt kinase inhibitor that shows pharmacodynamic inhibition of Akt signaling and robust antitumor activity in human cancer cells in vitro and in vivo. Our preclinical data provide a strong mechanistic rationale to evaluate GDC-0068 in cancers with activated Akt signaling. Clin Cancer Res; 19(7); 1760–72. ©2012 AACR.

Summary The enzyme glutaminase (GLS1) is currently in clinical trials for oncology, yet there are no clear diagnostic criteria to identify responders. The evaluation of 25 basal breast lines expressing GLS1, predominantly through its splice isoform GAC, demonstrated that only GLS1-dependent basal B lines required it for maintaining de novo glutathione synthesis in addition to mitochondrial bioenergetics. Drug sensitivity profiling of 407 tumor lines with GLS1 and gamma-glutamylcysteine synthetase (GCS) inhibitors revealed a high degree of co-dependency on both enzymes across indications, suggesting that redox balance is a key function of GLS1 in tumors. To leverage these findings, we derived a pan-cancer metabolic signature predictive of GLS1/GCS co-dependency and validated it in vivo using four lung patient-derived xenograft models, revealing the additional requirement for expression of GAC above a threshold (log2RPKM + 1 ≥ 4.5, where RPKM is reads per kilobase per million mapped reads). Analysis of the pan-TCGA dataset with our signature identified multiple indications, including mesenchymal tumors, as putative responders to GLS1 inhibitors.

Metabolic reprogramming in tumors represents a potential therapeutic target. Herein we used shRNA depletion and a novel lactate dehydrogenase (LDHA) inhibitor, GNE-140, to probe the role of LDHA in tumor growth in vitro and in vivo. In MIA PaCa-2 human pancreatic cells, LDHA inhibition rapidly affected global metabolism, although cell death only occurred after 2 d of continuous LDHA inhibition. Pancreatic cell lines that utilize oxidative phosphorylation (OXPHOS) rather than glycolysis were inherently resistant to GNE-140, but could be resensitized to GNE-140 with the OXPHOS inhibitor phenformin. Acquired resistance to GNE-140 was driven by activation of the AMPK-mTOR-S6K signaling pathway, which led to increased OXPHOS, and inhibitors targeting this pathway could prevent resistance. Thus, combining an LDHA inhibitor with compounds targeting the mitochondrial or AMPK-S6K signaling axis may not only broaden the clinical utility of LDHA inhibitors beyond glycolytically dependent tumors but also reduce the emergence of resistance to LDHA inhibition.

Although targeting cancer metabolism is a promising therapeutic strategy, clinical success will depend on an accurate diagnostic identification of tumor subtypes with specific metabolic requirements. Through broad metabolite profiling, we successfully identified three highly distinct metabolic subtypes in pancreatic ductal adenocarcinoma (PDAC). One subtype was defined by reduced proliferative capacity, whereas the other two subtypes (glycolytic and lipogenic) showed distinct metabolite levels associated with glycolysis, lipogenesis, and redox pathways, confirmed at the transcriptional level. The glycolytic and lipogenic subtypes showed striking differences in glucose and glutamine utilization, as well as mitochondrial function, and corresponded to differences in cell sensitivity to inhibitors of glycolysis, glutamine metabolism, lipid synthesis, and redox balance. In PDAC clinical samples, the lipogenic subtype associated with the epithelial (classical) subtype, whereas the glycolytic subtype strongly associated with the mesenchymal (QM-PDA) subtype, suggesting functional relevance in disease progression. Pharmacogenomic screening of an additional ∼ 200 non-PDAC cell lines validated the association between mesenchymal status and metabolic drug response in other tumor indications. Our findings highlight the utility of broad metabolite profiling to predict sensitivity of tumors to a variety of metabolic inhibitors.

The phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt)/ mammalian target of rapamycin (mTOR) signaling pathway is a major regulator of tumor cell growth, proliferation and survival. Dysregulation of the PI3K/Akt/mTOR signaling pathway through multiple mechanisms has been described in solid tumor malignancies, including activating and transforming “hotspot” mutations as well as amplification of PIK3CA that encodes the p110 alpha subunit of PI3K. Hotspot mutations of PIK3CA mutation are frequently observed in breast cancer with a prevalence of approximately 30%. GDC-0077 is a potent inhibitor of PI3K alpha (IC50 = 0.038 + 0.003 nM) and exerts its activity by binding to the ATP binding site of PI3K, thereby inhibiting the phosphorylation of PIP2 to PIP3. Biochemically, GDC-0077 is more than 300-fold selective over other Class I PI3K isoforms such as beta, delta, and gamma and more than 2000 fold more selective over PI3K class II and III family members. Importantly, GDC-0077 is more selective for mutant versus wild-type PI3K alpha in cell based assays. Compared to the PI3K inhibitor, taselisib, the improved biochemical selectivity of GDC-0077 against PI3K delta is demonstrated in human CD69+ B-cells, which are primarily dependent on PI3K delta for proliferation and survival and were more sensitive to taselisib than GDC-0077. Mechanism of action studies indicate that GDC-0077 induces depletion of mutant PI3K alpha protein resulting in reduction of PI3K pathway biomarkers such as pAkt and pPRAS40, inhibition of cell proliferation and increased apoptosis in human PIK3CA mutant breast cancer cell lines to a greater extent when compared to PIK3CA wild-type cells. In vivo, daily oral treatment with GDC-0077 in cell-culture-derived and patient derived PIK3CA mutant breast cancer xenograft models, resulted in tumor regressions, induction of apoptosis and a reduction of pAkt, pPRAS40, and pS6RP in a dose-dependent fashion. In vivo efficacy in a PIK3CA-mutant human breast cancer xenograft model was also improved when GDC-0077 was combined with standard-of-care therapies for hormone-receptor positive (HR+) breast cancer such as anti-estrogens (fulvestrant) or CDK4/6 inhibitor (palbociclib). Collectively, the preclinical data provide rationale for evaluating GDC-0077, a PI3K alpha mutant selective inhibitor, as a single agent and in combination with standard-of-care endocrine and targeted therapies that may provide additional benefit to patients that harbor PIK3CA mutations. Citation Format: Kyle Edgar, Emily Hanan, Steven Staben, Stephen Schmidt, Rebecca Hong, Kyung Song, Amy Young, Patricia Hamilton, Alfonso Arrazate, Cecile de la Cruz, Marcia Belvin, Michelle Nannini, Lori S. Friedman, Deepak Sampath. Preclinical characterization of GDC-0077, a specific PI3K alpha inhibitor in early clinical development [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 156. doi:10.1158/1538-7445.AM2017-156

Targeting cancer metabolism is currently being evaluated as a potential therapeutic target. Identification of tumor subtypes with specific metabolic requirements will enable the success of such therapies. Using cell line models of pancreatic ductal adenocarcinoma (PDAC), we conducted broad metabolite profiling and identified 3 metabolic subtypes through non-negative matrix factorization (NMF) consensus clustering. Among the most distinguishing features between the subtypes were levels of metabolites important for lipid synthesis, amino acid, and energy-related pathways, suggestive of distinct metabolic dependencies. Using RNAseq we show that these subtypes are defined at the transcriptional level and are associated with known subtypes of PDAC. In metabolic flux experiments, we also show that glucose and glutamine are differentially utilized by each subtype, and as a consequence, exhibit distinct sensitivity to inhibition of various metabolic pathways. Finally, through screening of a large panel of tumor cell lines with various metabolic inhibitors, we cross-validate our finding in PDAC and provide strong evidence that these same subtypes exist and can be identified in other tumor indications. Our findings highlight the utility of broad metabolite profiling to predict sensitivity to metabolic inhibitors in cancer cells. Citation Format: Anneleen Daemen, David Peterson, Nisebita Sahu, Ron McCord, Kaska Kowanetz, Anna Hitz, Xiangnan Du, Bonnie Liu, Min Gao, John Moffat, Rebecca Hong, Deepak Sampath, Mark Merchant, Thomas O'Brien, Bob Yauch, Jeff Settleman, Jing Qing, Georgia Hatzivassiliou, Marie A. Evangelista. Identification of distinct metabolic subtypes within pancreatic ductal adenocarcinoma through broad metabolite profiling. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1430. doi:10.1158/1538-7445.AM2014-1430

Background: Akt, a serine/threonine protein kinase, is a key signaling node in the PI3K/Akt/mTOR pathway and plays an essential role in regulating tumor cell proliferation, migration and survival. PI3K/Akt signaling is up-regulated in gastric cancer, mainly due to loss of PTEN, and is associated with poor prognosis and chemoresistance. Chemotherapeutics such as doxorubicin and 5-FU/cisplatin have been shown to upregulate PI3K/Akt signaling in several solid cancers in response to DNA damage. We have previously reported on a potent highly selective ATP-competitive pan-Akt inhibitor, GDC-0068, which inhibits cell cycle progression and viability of cancer cell lines and xenografts driven by Akt signaling, including those with defects in the tumor suppressor PTEN, oncogenic mutations in PIK3CA, and amplification of HER2. Thus, we hypothesized that the combination of FOLFOX and GDC-0068 would induce anti-tumor activity in primary gastric cancer models. Methods: Gastric cancer cell lines (n=11) were treated with increasing concentrations of GDC-0068 and FOLFOX and evaluated for combinational efficacy over single agent via cell viability assay. For in vivo studies, primary gastric cancer xenograft models were implanted in immuno-compromised mice and treated with GDC-0068 and FOLFOX as single agents or in combination. Results: In vitro, the combination of GDC-0068 and FOLFOX results in enhanced inhibition of tumor cell viability in the cell lines with activation of the PI3K pathway compared to either single agent alone. These results were recapitulated in vivo, in the primary xenograft models STO#240 and STO#182, where the combination of GDC-0068 and FOLFOX resulted in increased tumor growth inhibition or regressions compared to single agents alone. All combinations tested were well tolerated in vivo based on minimal changes in body weights. Conclusions: Our preclinical studies demonstrate improved anti-tumor efficacy when GDC-0068 is combined with FOLFOX in gastric cancer models in vitro and in vivo. The data supports the clinical development of GDC-0068 in combination with FOLFOX for the treatment of gastric cancer. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):B190. Citation Format: Ellen Ingalla, Rebecca Hong, Heidi Savage, Elizabeth Punnoose, Sandra Rost, Hartmut Koeppen, Deepak Sampath, Michelle A. Nannini. GDC-0068 is a novel and selective Akt inhibitor that enhances the efficacy of FOLFOX in primary gastric cancer models. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr B190.

Endocrine therapies such as letrozole or fulvestrant are commonly used treatment options for metastatic Hormone Receptor positive (HR+) breast cancer but patients ultimately relapse. Phosphatidylinositol 3-kinases (PI3K) regulate breast tumor cell growth, migration and survival. The alpha isoform of PI3K is frequently mutated and activated in HR+ breast cancer and has been implicated in resistance to endocrine therapies. PI3K inhibitors are therefore attractive for combination with endocrine therapies. GDC 0032 is an orally bioavailable, potent, and selective inhibitor of Class I PI3K alpha, delta, and gamma isoforms, with 30 fold less inhibition of the PI3K beta isoform relative to the PI3K alpha isoform. Preclinical data show that GDC-0032 has increased activity against PI3K alpha isoform (PIK3CA) mutant and HER2-amplified cancer cell lines. Single agent and combination studies were carried out to determine if GDC-0032 enhances the anti-tumor activity of endocrine therapies in human breast cancer models. MCF7 cells (Estrogen Receptor positive (ER+), PI3K alpha E545K mutant) were transfected with the aromatase gene and stable clones (MCF7-ARO) were selected that are capable of converting androstenedione to estrogen in culture. When grown in the presence of androstenedione, MCF7-ARO cells were more reliant on estrogen for growth. Under these conditions the cells were treated with GDC-0032 in combination with endocrine therapies and assayed for cellular viability, modulation of PI3K pathway and ER pathway markers and apoptosis induction. The combination of GDC-0032 and endocrine therapies decreased the cellular viability of MCF7-ARO cells and increased apoptosis relative to either single agent. We observed cross-talk between the PI3K and ER pathways that suggests a mechanism of action for the combination. The data provide rationale for evaluating GDC-0032 in combination with endocrine therapies for HR+ breast cancer treatment in the clinic. Citation Format: Jeffrey J. Wallin, Jane Guan, Rebecca Hong, Carol O'Brien, Timothy Wilson, Michelle Nannini, Deepak Sampath, Marcia Belvin, Lori S. Friedman. GDC-0032 PI3K inhibitor enhances the efficacy of endocrine therapies in breast cancer cells by differentially regulating the expression of ER-targeted genes and increasing apoptosis. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 546. doi:10.1158/1538-7445.AM2013-546